docs: gpio: add a new document to its index.rst

There's now a new ReST file. Add it to the index.rst file.

Fixes: ce7a2f77 ("docs: gpio: Add GPIO Aggregator documentation")
Fixes: 2fd1abe9 ("Documentation: gpio: add documentation for gpio-mockup")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/bad4d85c218d59c1bf69157df8e1012577680d88.1601616399.git.mchehab+huawei@kernel.orgSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/admin-guide/gpio/index.rst

@@ -9,6 +9,7 @@ gpio
 
     gpio-aggregator
     sysfs
+    gpio-mockup
 
 .. only::  subproject and html